One quasi-bug I've found with ATA's that they can only store 2 failover addresses. We have a Publisher and 2 Subscribers. So I configured one Subscriber as primary, the other Subscriber as the first backup and the Publisher as the third option, within the CCM group listing inside the device pool. However, that filled the ATA's failover list. Therefore, when I tested loss of WAN connectivity and all of the CUCM servers were unavailable, SRST, which was the 4th option, wasn't on the list, and the ATA's sat spinning through the primary and 2 backups.
So what I did was create a Device Pool just for ATA's, and only included one Subscriber as primary and the Publish as backup. Since I have done that, the ATA's have failed over to SRST just fine.